An image forming apparatus includes a process unit for forming a toner image on a sheet, and a fixing device for fixing the toner image to the sheet. The fixing unit includes a heat unit extending in an axial direction and provided with a heater, and an opponent member facing the heat unit in a direction perpendicular to the axial direction. The sheet on which the toner image has been formed is nipped between the heater unit and the opponent member while being conveyed, and is heated by the heat unit, to thus thermally fix the toner image to the sheet.
In such a conventional fixing device, end regions of the heat unit in the axial direction have a temperature higher than that of a central region of the heat unit in the axial direction. For example, in case of successive printing to a plurality of sheets whose width in the axial direction is smaller than the axial length of the heat unit, the plurality of sheets draw heat from the central region of the heat unit, whereas heat can be accumulated at the end regions of the heat unit because the plurality of sheets are not in contact with the end portions. Accordingly, temperature at the end regions is higher than that of the center region.
Japanese Patent Application Publication No. 2012-234067 discloses an image forming apparatus provided with a fan and an assembly for changing an opening width. The fan is provided in confrontation with the end region of the heat unit. The assembly is provided between the heat unit and the fan, and is formed with an opening allowing fan-air to pass therethough. A slide plate is slidably movable in an axial direction of the heat unit for changing a width of the opening to restrain temperature increase at the end region of the heat unit.
Further, Japanese Patent Application Publication No. 2001-142544 discloses an image forming apparatus provided with a cover at a position between a heat unit and a housing of the image forming apparatus in order to retain heat of the heat unit.